The present invention relates to a multiple-stage optical isolator which is positioned between a semiconductor laser as a light source and a light-transmitting part, for instance, in an optical transmission system to thus prevent a laser beam reflected by the optical transmission part from returning to the light source.
In the optical transmission, transmitted light rays which are emitted from a semiconductor laser as a light source are reflected by various kinds of optical transmission parts arranged on the transmission line and the reflected light rays often return to the semiconductor laser. The reflected light rays interfere with the light-emitting action of the semiconductor laser and often become a cause of noises. The optical isolator is provided in the course of the light transmission line to eliminate such noises. More specifically, the optical isolator permits the transmission of only the light rays progressing towards the direction of transmission, while preventing the reflected light rays which progress in the direction opposite to the light-transmitting direction from returning back to the semiconductor laser.
The optical isolators are divided into the polarization-dependent type optical isolator which is affected by the plane of polarization of the incident light rays and the polarization-independent type optical isolator which is not affected by the plane of polarization of the incident light rays. The polarization-dependent type optical isolator permits the transmission of only the light rays whose plane of polarization is adapted to the isolator and cannot transmit any light rays having other planes of polarization. Thus, the polarization-dependent type optical isolator exhibits a high optical insertion loss in the direction of transmission and accordingly, there has in general been used the polarization-independent type optical isolator which makes use of a birefringent crystal.
The polarization-independent type optical isolator separates and couples light rays by the action of a birefringent crystal so that it permits the transmission of only the light rays progressing along the direction of transmission, while keeping the reflected light rays which progress in the direction opposite to the light-transmitting direction away from the optical path to thus prevent the reflection back thereof to the semiconductor laser.
An article of Kok Wai Chang et al. (Optical Letters, 1990, April 15, Vol. 15, No. 8) discloses a multiple-stage polarization-independent type optical isolator. A light beam incident upon the optical isolator is divided into two beams and they progress while repeating the axial displacement due to the action of the birefringent crystal. These two light beams are different from one another in the number of the axial displacements within the optical isolator and accordingly, their optical path lengths till they are outputted from the optical isolator are in turn different from one another. For this reason, the optical isolator exhibits high polarization-dependency and polarization-mode dispersion.
Japanese Patent Provisional Publication No. Hei 4-264516 investigates a method for preventing any deterioration of optical characteristics of an optical isolator which comprises variously changing the thickness of each birefringent crystal incorporated into the optical isolator. However, the thickness ratio of the birefringent crystals incorporated therein is not specified and this accordingly, makes the structure of the isolator quite complicated and the assembly thereof requires a long period of time.